Bulk Mono-Crystalline Gallium-Containing Nitride and Its Application

ABSTRACT

Bulk monocrystalline gallium-containing nitride, grown on the seed at least in the direction essentially perpendicular to the direction of the seed growth, essentially without propagation of crystalline defects as present in the seed, having the dislocation density not exceeding 10 4 /cm 2  and considerably lower compared to the dislocation density of the seed, and having a large curvature radius of the crystalline lattice, preferably longer than 15 m, more preferably longer than 30 m, and most preferably of about 70 m, considerably longer than the curvature radius of the crystalline lattice of the seed.

TECHNICAL FIELD

The subject of this invention is a bulk mono-crystallinegallium-containing nitride for the use as the substrate for epitaxy inthe process of obtaining nitride semiconductor structures, as well as amethod of preparing the bulk mono-crystalline gallium-containing nitrideby using a combination of a flux method and a ammono method.

Group XIII element-containing nitrides (IUPAC, 1989) are a preciousmaterial for the opto-electronic industry.

Bulk mono-crystalline gallium-containing nitride is considered a perfectsubstrate for deposition of epitaxial layers of gallium nitride, whoseenergy gap may be used for producing laser diodes (LD) and bluelight-emitting diodes (LED). The condition however, which must be met sothat it could be used as the substrate for epitaxy, is its highcrystalline quality and low dislocation density of the single crystal.

Bulk mono-crystalline gallium-containing nitrides obtained through themethods used so far do not meet those requirements. Nevertheless, theexpected demand for the material of proper quality stimulates researchand technological progress in that field.

BACKGROUND ART

The authors of the publication WO 02/101120 disclosed the method ofcrystallization on the seed from the super-critical solution containinga nitride-containing solvent, preferably ammonia. By this method it ispossible to obtain bulk mono-crystalline gallium-containing nitrides ofa higher quality of parameters compared to the substrates used inindustry, which are obtained by deposition methods from the gaseousphase, such as HVPE and MOCVD or MBE, i.e., of lower dislocation densitythan those substrates. Mono-crystals obtained by the method as knownfrom the disclosure of WO 02/101120 show high increments in volume. Dueto equilibrium character of the crystallization process, a very highcrystalline quality of single crystals is attained from the solutioncontaining a supercritical gallium-containing solvent, compared to thematerials used industrially in various centers worldwide. The primaryadvantage of the technology known from WO 02/101120 is that it canassure a convenient range of pressures and temperatures, in which there-crystallization process of gallium-containing nitride from thesupercritical solution based on the nitride-containing solvent takesplace.

In course of further research and developmental work over the methoddisclosed in WO 02/101120 a number of factors, which had a limitingimpact on practical application of that method, were recognized andgradually the encountered barriers were surmounted, both in terms oftechnology and apparatuses. Some of such barriers are: a limitedavailability of the feedstock of desired purity, proper quality ofcrystalline seeds, selection of proper mineralizers and control of thegrowth rate of single crystals.

Other methods of synthesis of gallium-containing nitrides, such as HNP,are also known. By those methods gallium-containing single crystals ofvery high crystalline quality and low dislocation density are obtained.Unfortunately, because of the unsatisfactory size and irregular shape ofcrystals which are obtained thereby, they have not been used so far asthe material for the substrate for epitaxy in industrial production ofLEDs, LDs and other semiconductor structures. Also, parameters of theprocess, and in particular the necessity of using very high pressures,significantly limit the feasibility of obtaining crystals of a desiredsize by this method on an industrial scale.

The studies in that filed show promising results obtained with the useof flux methods of growth of gallium-containing nitrides from a galliummelt in the atmosphere of nitrogen. Those processes are attractiveindustrially because relatively low temperatures and pressures are used.

The basic starting materials for the process disclosed in WO 02/101120,i.e., the feedstock subject to re-crystallization and the seeds, wereobtained by HVPE method, according to which mono-crystalline galliumnitride layers from the gaseous phase are placed on hetero-seeds, inparticular on sapphire. As the result of differences between latticeconstant of the hetero-seed and the obtained bulk mono-crystallinegallium-containing nitride, as well as the result of differences inthermal expansion of both materials, bulk gallium-containing nitridesingle crystals, preferably gallium nitride single crystals, obtained byHVPE method have a disordered crystalline structure, which is reflected,e.g., in small radius of curvature of the obtained bulk mono-crystallinegallium-containing nitrides. The use of such single crystals ascrystallization seeds in the process of recrystallization ofmono-crystalline gallium-containing nitride from the supercriticalammonia-containing solution leads to propagation of crystalline defectsand surface dislocations in mono-crystalline gallium nitride layersobtained on such seeds. Additionally, different conditions of growth onthe gallium-terminated and nitrogen-terminated sides of seeds wereobserved on seeds in the form of wafers oriented perpendicularly to thec axis of the gallium nitride crystalline lattice.

The authors of the publication WO 03/035945 disclosed that it waspossible to effectively improve the quality of crystallization seeds byway of covering them with the ELOG structures having surfacessusceptible to lateral growth, i.e., toward the a axis of thecrystalline lattice of gallium-containing nitride, i.e., in accordancewith the methods of quality improvement of substrates for epitaxyobtained by the methods of growth from the gaseous phase. Considering,however, random arrangement of crystalline defects and surfacedislocations, it is impossible, on the seeds covered with ELOGstructures, to eliminate in sufficient degree the propagation ofcrystalline defects of the primary substrate, obtained by HVPE method,to mono-crystalline gallium nitride layers deposited from thesupercritical solution based on nitride-containing solvent. Surfacessusceptible to lateral growth arranged in small distances from eachother are separated by strips grown directly on the primary substrate.It must be noted at this point that multiplied and alternate depositionof the ELOG structures on crystalline seeds cannot be taken into accountmainly because of high costs.

DISCLOSURE OF INVENTION

A first object of the present invention is to provide bulkmono-crystalline gallium-containing nitrides grown on the seed,basically without propagation of crystalline defects as present in theseed as well as to assure the substrate for epitaxy from bulkmono-crystalline gallium-containing nitride having improved crystallineproperties and reduced dislocation density.

A second object of the present invention is also to provide longerlifetime of semiconductor structures, deposited on a new improvedsubstrate for epitaxy.

A third object of the present invention is also to provide a method ofpreparing the bulk mono-crystalline gallium-containing nitride by acombination of a flux method and an ammono method.

The inventors unexpectedly discovered that it was possible to achievethose aims by way of preparing the process according to the presentinvention, which assures bulk mono-crystalline gallium-containingnitrides of required size, geometrical parameters and desiredcrystalline quality. Consequently, it was possible to obtain a substratefor epitaxy of any orientation and sufficiently high crystallographicquality.

According to the invention, bulk mono-crystalline gallium-containingnitride, grown on the seed at least in the direction essentiallyperpendicular to the direction of the seed growth, essentially withoutpropagation of crystalline defects as present in the seed, ischaracterized in that its dislocation density does not exceed 10⁴/cm²and is considerably lower compared to the dislocation density of theseed, and has a large curvature radius of the crystalline lattice,preferably longer than 15 m, more preferably longer than 30 m, and mostpreferably of about 70 m, which is considerably longer than thecurvature radius of the crystalline lattice of the seed.

According to the invention, bulk mono-crystalline gallium-containingnitride grown on the seed, at least in the direction essentiallyperpendicular to the direction of seed growth, essentially withoutpropagation of crystalline defects as present in the seed, has the FWHMof X-ray rocking curve from (0002) plane, preferably below 40 arcsec(for Cu K α1) and considerably lower than the FWHM of the seed withsimultaneous large curvature radius of the crystalline lattice,preferably longer than 15 m, more preferably longer than 30 m, and mostpreferably of about 70 m, which is considerably longer than thecurvature radius of the crystalline lattice of the seed.

The single crystal according to the invention is preferably doped withdonor-type and/or acceptor-type, and/or magnetic-type dopants, at theconcentration from 10¹⁷/cm³ to 10²¹/cm³, and comprises n-type, p-type orcompensated (semi-insulating) material.

Preferably, the single crystal according to the invention is grown inthe environment and in the conditions in which the growth rate in thedirection perpendicular to the c axis, and in particular parallel to thea axis, is the same or higher than the growth rate in the directionparallel to the c axis of the single crystal.

The single crystal according to the invention is preferably a galliumnitride single crystal.

According to the invention, it is possible to make a wafer of anyorientation, with polar or non-polar planes, obtained as a singlecrystal according to the invention or cut out from such single crystal,whereas the cut is made in a desired direction with respect to thedirection of growth of the single crystal.

The wafer according to the invention has preferably its surfacedislocation density additionally reduced as the result of slicing in thedirection essentially parallel to the direction of the single crystalgrowth.

The wafer according to the invention has preferably non-polar surfaces,suitable for further processing.

The wafer according to the invention has preferably polar surfaces,suitable for further processing.

According to the invention the wafer may be used as a substrate forepitaxial deposition of semiconductor structures from Group XIIIelement-containing nitrides.

The invention also comprises the substrate for epitaxial deposition ofsemiconductor structures from Group XIII element-containing nitrides,which is obtained as single crystal according to the invention or is thewafer according to the invention, and in particular, which is suitablefor production of semiconductor structures requiring a nitride substrateof sufficiently low surface dislocation density, especially at GroupXIII element-terminated side and has epitaxial surface not smaller than100 mm², preferably not smaller than 450 mm².

The invention covers also semiconductor structures, which are obtainedon the substrate.

Due to preparation of the solution according to the invention it ispossible to assure homogenous bulk mono-crystalline gallium-containingnitride, especially gallium nitride, of exceptional crystallineparameters and exceptionally low surface dislocation density, which meetthe requirements of the optoelectronic industry.

Mono-crystalline gallium-containing nitride according to the inventionhas unusual dimensions, regular shape, and at the same time excellentcrystalline properties, adjusted to the technological requirements ofthe optoelectronic industry.

In a particularly preferable example of the invention, bulkmono-crystalline gallium-containing nitride has the assumed parametersof electrical conductivity. This feature of the substrate for epitaxyobtained from gallium-containing nitride single crystals will make itpossible to change laser structures and considerably increase the numberof such structures per substrate.

At the same time, it should be stressed that the solution according tothe invention is also preferable in terms of costs.

The enclosed drawing FIG. 1 shows a schematic diagram of the cruciblefor the growth of mono-crystalline gallium-containing nitride by theflux method, FIG. 2 shows the diagram of temperature changes in time inthe examples 4-9, FIG. 3 shows the cross-section view of the autoclaveand the set of furnaces used in the method of growth from thesupercritical ammonia-containing solution, and FIG. 4 shows aperspective view of an apparatus for obtaining mono-crystallinegallium-containing nitride according to the invention.

In the description below the following terms and definitions shall havethe following meaning unless otherwise specified.

Autoclave, regardless of its from, includes a closed reaction chamber,in which the crystallization process from fluid phase in theaforementioned range of temperature and pressure is carried out. Forcrystallization from supercritical ammonia-containing solution it isconvenient to use a device presented schematically in FIG. 7 and FIG. 8,discussed in detail further in the text.

Gallium-containing nitride is a chemical compound containing in itsstructure at least one atom of gallium and one atom of nitrogen. Itincludes, but is not restricted to, a binary compound—GaN, a ternarycompound—AlGaN, InGaN or a quaternary compound AlInGaN, preferablycontaining a substantial portion of gallium, anyhow at the level higherthan dopant content. The composition of other elements with respect togallium in this compound may be modified in its structure insofar as itdoes not collide with the selected crystallization technique.

Crystallographic directions c, a or m refer to c, a or m directions ofhexagonal lattice, having the following Miller indices: c—[0001], a—[1120], m—[1 100].

Crystallization from melt refers to crystallization by flux method.

Gallium-containing feedstock is gallium-containing nitride or itsprecursor. As a feedstock, GaN obtained by various methods may be used,among others by flux methods, HNP method, HVPE methods. Moreover,polycrystalline GaN obtained by reaction of metallic gallium withsupercritical ammonia-containing solution may be used.

HVPE (Halide Vapor Phase Epitaxy) method refers to a method ofdeposition of epitaxial layers from gaseous phase, in which (in the caseof Group XIII—element nitrides) halides of Group XIII metals and ammoniaare used as substrates.

MBE (Molecular Beam Epitaxy) method refers to a method of obtainingepitaxial layers of atomic thickness by depositing molecules fromso-called “molecular beam” on a substrate.

MOCVD (Metallo-Organic Chemical Vapor Deposition) method refers to amethod of deposition of epitaxial layers from gaseous phase, in which(in the case of nitrides) ammonia and metallo-organic compounds ofgallium are used as substrates.

Crystallization methods from fluid phase in this application refer tocrystallization from supercritical ammonia-containing solution or toflux method.

Flux methods of obtaining crystalline gallium nitride mean a group ofmethods, in which said azide is obtained as the result of a chemicalreaction between liquid mixture of metals (melt) and nitrogen-containinggas (in particular, it may be gaseous nitrogen or a mixture of nitrogenand ammonia). The melt contains among others gallium and flux. Ofcourse, this process proceeds at appropriate temperature and pressureconditions. In the case sodium, which is a well known flux, typicaltemperature of the process is ca. 600-800° C., while typical pressure isca. 5 MPa.

Mineralizer is a substance introducing into the supercriticalammonia-containing solvent one or more Group I element (alkali metal)ions, supporting dissolution of feedstock (and gallium-containingnitride).

Supercritical ammonia-containing solvent is a supercritical solventconsisting at least of ammonia, which contains one or more types of ionsof Group I elements (alkali metals), supporting dissolution ofgallium-containing nitride. Supercritical ammonia-containing solvent mayalso contain derivatives of ammonia and/or mixtures thereof, inparticular—hydrazine.

Supercritical ammonia-containing solution means a solution obtained asthe result of dissolution of gallium-containing feedstock in thesupercritical ammonia-containing solvent.

Bulk mono-crystalline gallium-containing nitride means amono-crystalline substrate in the form of gallium-containing nitride, onwhich opto-electronic devices may be obtained, such as: light-emittingdiodes (LED) or laser diodes (LD) by MOCVD method or by the methods ofepitaxy growth such as HVPE.

Crystallographic planes C, A or M refer to C-, A- or M-plane surfaces ofhexagonal lattice, having the following Miller indices: C—(0001), A—(1120), M—(1 100). The surfaces are perpendicular to the correspondingcrystallographic directions (c, a and m).

Polar or non-polar further-processable surface—means a surface suitablefor epitaxial deposition of nitride layers, whereon it is possible toproduce at least one optoelectronic device. Such surface should have thesize sufficient for epitaxy by MOCVD, MBE methods or other method ofepitaxial deposition of nitride layers, preferably larger than 10 mm²,and most preferably larger than 10 mm².

Polar or non-polar crystalline surface: In crystals of Group XIIIelement nitrides of wurtzite structure, the crystalline planes parallelto the c axis of the crystal (and crystal surfaces containing thoseplanes) are called non-polar surfaces, whereas, crystalline planesperpendicular to the c axis of the crystal (and crystal surfacescontaining those planes) are called non-polar surfaces.

Precursor of gallium-containing nitride is a substance or a mixturecontaining at least gallium and optionally containing elements of GroupI (alkali metals), elements of Group II (alkali earth metals), elementsof Group XIII (group numbers according to IUPAC 1989), nitrogen and/orhydrogen, and metallic gallium, its alloys or metallic compounds,hydrides, amides, imides, amido-imides and azides, which may formgallium compounds soluble in the supercritical ammonia-containingsolvent as defined below.

Super-Saturation

If concentration of soluble gallium compounds in the supercriticalammonia-containing solution is higher than solubility ofgallium-containing nitride in specific physicochemical conditions, thensuper-saturation of the supercritical ammonia-containing solution withrespect to gallium-containing nitride in those conditions can be definedas the difference between the actual concentration and the solubility.While dissolving gallium-containing nitride in a closed system it ispossible to obtain the super-saturation state, for example by increasingtemperature or decreasing pressure.

Diffusion process in this application means a process of crystal growth,in which the transport between feedstock and seeds proceeds essentiallyby diffusion.

Convection process in this application means a process of crystalgrowth, in which the transport between feedstock and seeds proceedsessentially by convection.

Solubility

Our experiences show that the state of equilibrium may be achievedbetween the solid (gallium-containing nitride) and the supercriticalsolution at sufficiently high temperature and pressure. Therefore,solubility of gallium-containing nitride may be defined as theequilibrium concentration of soluble gallium compounds obtained in theabove mentioned process of dissolution of gallium-containing nitride. Inthis process, the equilibrium concentration, i.e. solubility, may becontrolled by changing the composition of solvent, temperature and/orpressure.

Dissolution of gallium-containing feedstock means either reversible orirreversible process of forming out of said feedstock gallium compoundssoluble in the supercritical solvent, for example gallium-complexcompounds. Gallium complex compounds are complex chemical compounds, inwhich an atom of gallium is a coordination center surrounded by ligands,such as ammonia molecules (NH3) or their derivatives, like NH2-, NH2-,etc.

Selective crystallization on a seed means a process of crystallizationtaking place on the surface of the seed, in the absence of spontaneouscrystallization or with spontaneous crystallization occurring in anegligible degree. This process is indispensable for achieving the aimof the present invention, i.e. obtaining bulk single crystals ofgallium-containing nitride, and at the same time it is an essentialelement of the present invention.

Spontaneous crystallization from the supersaturated supercriticalammonia-containing solution means any undesirable process of nucleationand growth of the gallium-containing nitride crystals taking place atany site within the autoclave except on the surface of the seed. Thedefinition also includes growth on the surface of the seed, in which thegrown crystal has an orientation different from that of the seed.

Melt in this application means a mixture of molten metals.

Group XIII element-terminated side, Ga-terminated side, N-terminatedside: In the crystals having the wurtzite structure one can distinguisha crystalline direction (crystalline axis) denoted as a and anothercrystalline direction—c—which is perpendicular to a. In the crystals ofGroup XIII element nitrides, having the wurtzite structure, thecrystalline planes perpendicular to the c axis are not equivalent. In isa habit to call them Group XIII element-terminated side andnitrogen-terminated side or the surface having Group XIII elementpolarity or nitrogen polarity, respectively. In particular, in the caseof mono-crystalline gallium nitride one can distinguishgallium-terminated side (Ga-side) and nitrogen-terminated side (N-side).These sides have different chemical and physical properties (eg.susceptibility to etching or thermal durability). In the methods ofepitaxy from the gaseous phase the layers are deposited on the GroupXIII element-terminated side.

Temperature and Pressure of the Reaction

In the practical examples presented in the present specificationtemperature measurements inside the autoclave have been performed whenthe autoclave was empty, i.e. without the supercriticalammonia-containing solution. Thus, the temperature values cited in theexamples are not the actual temperature values of the process carriedout in the supercritical state. Pressure was measured directly orcalculated on the basis of physical and chemical data forammonia-containing solvent at selected process temperature and thevolume of the autoclave. In the case of flux methods the temperature wasmeasured inside the autoclave, but outside the crucible. Nevertheless,the values of temperature given in this application should be very closeto actual temperature values in the melt contained in the crucible.

Flux means a substance added to the reaction environment in fluxmethods, which helps maintain reactants in liquid phase throughout theprocess.

Chemical transport of gallium-containing nitride in the supercriticalsolution means a continuous process involving dissolution of agallium-containing feedstock in the supercritical solution, circulationof the soluble gallium compounds within the solution and crystallizationof gallium-containing nitride from the super-saturated supercriticalsolution. Generally, chemical transport may be caused by temperaturedifference, pressure difference, concentration difference, or otherchemical or physical differences between the dissolved feedstock and thecrystallization product. According to the present invention, bulkmono-crystalline gallium-containing nitride may be obtained in effect ofchemical transport between the dissolution and crystallization zones ofthe autoclave, established by means of temperature difference betweenthe two zones, whereas the temperature of crystallization zone should behigher than the temperature of dissolution zone.

Temperature and pressure coefficient of solubility (TCS and PCS)Negative temperature coefficient of solubility means that the solubilityis a decreasing function of temperature if all other parameters are keptconstant. Similarly, positive pressure coefficient of solubility meansthat, if all other parameters are kept constant, the solubility is anincreasing function of pressure. Our research allows to state thatsolubility of gallium-containing nitride in the supercriticalammonia-containing solvent, at least in the temperature range from 300to 550° C., and pressure from 100 to 550 MPa, shows a negativetemperature coefficient (negative TCS) and a positive pressurecoefficient (positive PCS).

Lateral growth in this patent application refers to bulk growth on aseed in the direction perpendicular to the original direction of seedgrowth. In contrast to ELOG (Epitaxial Lateral Overgrowth), the lateralgrowth is definitely macroscopic (of the order of dimensions of the seedor even larger) and it is the aim of the process. Moreover, theprojection of a laterally grown crystal in the direction parallel to theoriginal direction of seed growth goes remarkably beyond the projectionof the seed used. In the case of ELOG (Epitaxial Lateral Overgrowth),these two projections are essentially identical.

ELOG (Epitaxial Lateral Overgrowth) is a method of crystal growth fromgaseous phase or from supercritical ammonia-containing solution, inwhich crystals are grown on a special substrate. In the case of galliumnitride crystals, a matrix of parallel ridges (several microns high andseveral microns wide), having surfaces susceptible to lateral growth, iscreated on the surface of the substrate. Typically, gallium nitridecrystals are grown in the c direction. The ridges are then created alongthe m direction and the surfaces susceptible to lateral growth coincidewith A-planes. In this case, lateral growth is limited to several orseveral dozen microns and it is finished as soon as the space betweenthe ridges becomes overgrown by the arising crystal. Next, the principalgrowth of bulk crystal proceeds along the c direction. This way some ofthe dislocations present in the substrate can be prevented frompenetrating into the arising crystal.

Seed is crucial for obtaining a desired bulk gallium-containing nitridemonocrystals in a process according to the present invention. In view ofthe fact that the quality of the seed is crucial for the crystallinequality of the bulk gallium-containing nitride monocrystals obtained bythe process according to the present invention, the seed selected forthe process should have possibly high quality. Various structures orwafers having a modified surface can also be used. For example astructure having a number of surfaces spaced adequately far from eachother, arranged on a primary substrate and susceptible to the lateralovergrowth of crystalline nitrides may be used as a seed. Moreover, aseed having a homoepitaxial surface, exhibiting n-type electricconductivity, for example doped with Si, may be used. Such seeds can beproduced using processes for gallium-containing nitride crystal growthfrom gaseous phase, such as HVPE or MOCVD, or else MBE. Doping with Siduring the growth process at the level of 1016 to 1021/cm2 ensuresn-type electric conductivity. Moreover, a composite seed may be used andin such seed directly on a primary substrate or on a buffer layer madefor example of AlN—a layer made of GaN doped with Si may be deposited

DETAILED DESCRIPTION OF THE INVENTION

Having analyzed their own experience in the scope of applying the methoddisclosed in WO 02/101120, and verified through tests the reports onavailable methods of obtaining gallium-containing nitride in themono-crystalline form, the inventors discovered that the growth of bulkmono-crystalline gallium-containing nitride proceeds at various rates invarious environments, and additionally at various rates in the directionof various axes of the hexagonal wurtzite-type crystalline lattice, inwhich gallium nitride and other gallium-containing nitride arecrystallized. This information is directly based on the shape ofgallium-containing nitride crystals obtained as the result of thespontaneous crystallization in the processes of obtaining that kind ofnitride single crystals.

The fact that spontaneous crystals obtained by the method, as disclosedin WO 02/101120, are in the form of needles of hexagonal section provesthat there is a preferable growth toward the c axis of the crystallinelattice of gallium containing nitride.

The low-temperature and low-pressure flux method of obtaining GaN fromGa—Na and Ga—Li melts in the nitrogen atmosphere is also known.Preferable pressure and temperature parameters as required in thoseprocesses caused that the authors of this invention became interested inthat process as a potential process allowing them to obtain thefeedstock and, possibly, crystallization seeds for the crystallizationprocess from the supercritical—preferablyammonia-containing—environment, of gallium-containing nitride solution.

The process of obtaining gallium nitride from the Ga—Na alloy melt ishowever arduous in technological terms because of the reactivity ofsodium with respect to humidity, high pressure of sodium vapors in theprocess conditions and its sublimation deposition in cooler parts of thereactor. Those properties of sodium as flux cause that it is difficultto apply this method industrially.

The publication Youting Song et. al. in the Journal of Crystal growth247 (2003) 275-278, reports on crystallization of GaN by flux methodwith the use of lithium as flux (temperature of about 800° C., pressure0.2 MPa, the duration of the process 120-180 hours). This report provesthat the use of lithium as flux makes it possible to obtain GaN in lessdrastic conditions, however an amount of spontaneously formed crystalsis still unsatisfactory.

Observations and experiences which have been carried out in terms ofapplication of the flux technology for obtaining the feedstock or seeds,show that flux methods allow for obtaining spontaneousgallium-containing nitride crystals in the form of hexagonal wafers ofhigh crystalline quality and low surface dislocation density. The shapeof crystals formed spontaneously (without seeds) shows that there is apreferable growth of gallium-containing nitride—in the processconditions—in directions perpendicular to the c axis of thegallium-containing nitride crystalline lattice.

Like in the flux method, which allows the crystal to grow in directionsperpendicular to the c axis, the present research on the method ofgrowth from supercritical ammonia-containing solution show that thecrystal growth by this method in the direction perpendicular to the caxis is also possible although it is geometrically limited and may bevery slow.

This means that the volume parameters of gallium-containing nitridesingle crystal obtained by the growth method from the supercriticalammonia-containing solution are, on the one hand, determined by thedimensions, shape and orientation of the seed, and on the other hand, bythe duration of the process and feedstock reserves in the system.

As the observations described above show, the growth ofgallium-containing nitride single crystal according to all known methodstakes place at least partially in the direction close to the directionof the growth of the seed crystal obtained by the same or differentmethod, which unfortunately means that at least partial propagation ofcrystalline defects as present in seeds takes place in the depositedmono-crystalline gallium-containing nitride layers.

It has been recently discovered that it is possible to obtaingallium-containing nitride crystals of essentially higher qualityparameters than parameters of the seed crystal through selection of theconditions in which the process of gallium-containing nitridecrystallization is carried out. At the same time, while aiming to assureproper dimensions of gallium-containing nitride the authors of thisinvention put appropriate focus on obtaining a crystal seed of properdimensions.

The bulk mono-crystalline gallium-containing nitride according to theinvention may be obtained by way of controlled growth of a singlecrystal in a desired direction in the process which comprises the stepof growth from the liquid phase in the direction perpendicular to thegrowth of mono-crystalline seed from gallium-containing nitride duringthe phase of its obtaining.

The single crystal growth from the liquid phase may be carried out byflux method with use of flux assuring liquidity of the system, in therange of temperature from 300° C. to 950° C.

The growth by flux method from the Ga—Li melt, which optionally containsan additional flux X, selected from the group consisting of Bi, In, K,Na, Pb, Rb, Sb, Sn and Te is preferable, whereas the molar ratio ofX:Ga:Li is from 0.5:1.0:1.5 to 1.5:1.0:2.5.

Preferably, with the use of flux method, the growth is carried out attemperature from 700° C. to 850° C., at pressure of nitrogen from 2.0 to2.5 MPa, and optionally with addition of crystalline gallium-containingnitride.

Preferably, in the phase in which the melt is heated to the desiredtemperature, the protective atmosphere of inert gas, preferably argon,is used. Next, nitrogen is added to the system and the growth of singlecrystal is carried out on the seed while the temperature gradient ismaintained within the melt, whereas the seed is placed in thelower-temperature zone.

It is recommended that, after the crystal growth by flux method iscompleted, the melt should be initially cooled slowly, and then, cooledfast to the ambient temperature.

Preferably, in the atmosphere of inert gas, it is possible to obtainheterogeneous Ga melt with fluxes by way of heating everything as longas the average temperature of the melt is above 700° C., and followed bystabilization of the melt, the atmosphere is changed, by replacing inertgas with nitrogen under pressure from 2.0 to 2.5 MPa, and next, in thesteady conditions the growth of crystals is carried out, and after thegrowth is completed the obtained crystals may be gradually taken outfrom the melt in the process conditions or everything may be cooled downas described above, and finally the obtained single crystals areseparated by dissolution of the solidified melt.

While heating the Li—Ga melt in zones, the temperature gradient is keptaround the melt, and the seeds are placed in a cooler zone.

It should be noted that in course of the diffusion process, thetemperature in the heating phase in the inert gas atmosphere is keptlower in the undersurface zone and higher in the bottom zone, whereasafter the atmosphere is changed into nitrogen, the temperature gradientis reversed.

Alternatively, in course of the convection process, additional feedstockis used—as internal source of nitrogen in the melt—in the form ofcrystalline nitride containing lithium, gallium or metal of the groupcomprising Bi, In, K, Na, Pb, Rb, Sb, Sn or Te, which is an additionalflux, and that additional feedstock is brought to the liquid phase byheating everything to the average temperate as specified above, whereasthe zone, in which that additional material was placed—to thetemperature higher by several dozen degrees centigrade.

Preferably, in both versions of the flux process, the temperaturedifference between the zones is kept at the level of several dozendegrees centigrade.

According to this invention, the growth of single crystals from theliquid phase may be also carried out by the growth method in thesupercritical solution in nitrogen-containing solvent, preferably in thesupercritical ammonia-containing solution.

According to this method, the system comprises in the crystallizationstage the gallium-containing feedstock, preferably crystalline galliumnitride, Group I elements and/or their mixtures, and/or their compounds,especially those containing nitrogen and/or hydrogen, preferably azides,optionally with addition of Group II elements and/or their compounds,which form the mineralizer, and the mineralizer together with ammoniaare used as ammonia-containing solvent. Crystallization of desiredgallium-containing nitride is carried out on the surface of the seed, atcrystallization temperature higher and/or under crystallization pressurelower than the temperature and pressure of dissolution of the feedstock.There are two temperature zones. There is feedstock in the dissolutionzone, and at least one seed is in the crystallization zone, whereas thedissolution zone is located above the crystallization zone and the massis transported from the dissolution zone to the crystallization zone.

Preferably, the temperature difference between the dissolution zone andthe crystallization zone is from 1° C. to 150° C., preferably from 10°C. to 100° C., and in the crystallization zone the temperature is notlower than 350° C., preferably not lower than 400° C., and mostpreferably ranges from 500° C. to 550° C.

Bulk mono-crystalline gallium-containing nitride according to theinvention is preferably obtained by growth in the directionperpendicular to the direction of the seed growth from mono-crystallinegallium-containing nitride of the same chemical composition by thegrowth method based on supercritical ammonia-containing solution.

Preferably, bulk mono-crystalline gallium-containing nitride accordingto the invention is obtained by way of controlled growth of singlecrystal in a desired direction, as the result of at least one phase ofgrowth in the direction perpendicular to the c axis of the singlecrystal in the liquid phase from the gallium-containing melt, based onlithium, and at least one phase of growth in the direction parallel tothe c axis of the single crystal in the supercritical ammonia-containingsolution, whereas feedstock and seed are used in each of those phases,and optionally the growth stages in the direction perpendicular to the caxis and in the direction along the c axis are repeated until desireddimensions of the single crystal are obtained along at least one of itsaxis.

Preferably, the seed used for obtaining bulk mono-crystallinegallium-containing nitride according to the invention is in the form ofgallium-containing nitride single crystal in the form of a waferoriented perpendicularly to the c axis of the single crystal, obtainedby the crystallization method from the gaseous phase, with the surfacedislocation density not larger than 10⁸/cm², and first, this seed iscovered with the gallium-containing nitride layer of a desired thicknessin the direction parallel to the c axis of single crystal and with thesurface dislocation density in the range from 10⁴/cm² to 10⁶/cm² by thegrowth method from the supercritical ammonia-containing solution, andthen the growth of that wafer is carried out in the directionperpendicular to the c axis by flux method, and next, it is possible todeposit another layer of gallium-containing nitride on the wafer grownperpendicularly towards the c axis of the single crystal while carryingout the growth from the supercritical ammonia-containing solution towardthe c axis.

Gallium-containing nitride crystal obtained by HNP method of very lowsurface dislocation density may be also used as a seed, which is, next,grown in the desired direction: in the direction perpendicular to the caxis by flux method, whereas in the direction along its c axis—by themethod of growth from the supercritical ammonia-containing solution,depending on the initial shape of the seed, and after a single crystalof desired dimensions is attained, the wafer of desired orientation iscut out from it, and afterwards it is possible to repeat the growthphase by flux method and/or growth method from the supercriticalammonia-containing solution.

Moreover, the gallium-containing nitride single crystal obtained in theform of hexagonal wafer by flux method as the result of spontaneouscrystallization may be used as a seed. Then, the growth of such seed iscarried out along the c axis by the growth method from the supercriticalammonia-containing solution, and after a single crystal of desireddimensions is attained, the wafer of desired orientation is cut out fromit, and afterwards it is possible to repeat the growth phase by fluxmethod and/or growth method from the supercritical ammonia-containingsolution.

According to another preferable embodiment of this invention, bulkmono-crystalline gallium-containing nitride is obtained through thecontrolled growth of single crystal in the desired direction, whichcomprises at least one step of growth in the direction perpendicular tothe c axis of the single crystal and at least one step of growth in thedirection parallel to the c axis of the single crystal in thesupercritical ammonia-containing solution with the use of feedstock andseed.

Usually, for obtaining bulk mono-crystalline gallium-containing nitrideaccording to the invention, gallium-containing nitride wafers obtainedby HVPE method are used as seeds.

Preferably, however, a gallium-containing nitride single crystal is usedas a seed, in the form of a wafer with at least one non-polar plane,obtained as single crystal or cut out from the single crystal obtainedby the crystallization method from the gaseous phase, or more preferablyby the growth method from the supercritical ammonia-containing solution,and afterwards the growth of that wafer is carried out in the directionperpendicular to the c axis of the single crystal by the flux methodand/or by the growth method from the supercritical ammonia-containingsolution.

Bulk mono-crystalline gallium-containing nitride according to theinvention may be doped with donor and/or acceptor and/or magnetic-typedopants, at the concentration from 10¹⁷/cm³ to 10²¹/cm³. As the resultof doping the gallium-containing nitride according to the invention isn-type material, p-type material or compensated material(semi-insulating).

Preferably, bulk mono-crystalline gallium-containing nitride is,according to this invention, in the form of gallium nitride.

Preferably, proper dimensions of the seed for obtaining bulkmono-crystalline gallium-containing nitride are assured by subjectingthe seed to the initial processing consisting in the growth alternatelyin the direction parallel to the c axis and in the directionperpendicular to the c axis of the gallium-containing nitridecrystalline lattice. The alternate growth of the crystal in thepredetermined directions is carried out through the growth alternatelyby the flux method from the Ga—Li melt in the direction perpendicular tothe c axis and by the growth method from the supercriticalammonia-containing solution in the direction parallel to the c axis.Alternatively, one of those methods is used, and in subsequent stagesthe growth planes in the desired direction are uncovered alternately,and simultaneously the growth in the direction perpendicular to it isreduced.

The examples of how to reduce the growth of gallium-containing nitridesingle crystals in the desired directions were disclosed in thepublication WO 03/035945.

The results of tests on single crystals according to this inventionconfirm that it is possible to achieve a very high crystalline qualityof mono-crystalline gallium-containing nitride based on mono-crystallinegallium-containing nitride wafers obtained by growth methods from thegaseous phase, especially by HVPE method, if in the subsequent steps ofthe method of obtaining mono-crystalline gallium-containing nitrideaccording to the invention the growth is carried out in the directionsperpendicular to the c axis. The wafers obtained in this way have verylarge curvature radius, longer than 15 m, more preferably longer than 30m, and most preferably above 70 m, whereas the curvature radius ofsingle crystals grown in the same direction as the direction of seedgrowth (parallel to the c axis) has a typical value of about 2-15 m. Atthe same time, the FWHM of single crystals according to the invention ispreferably below 40 arcsec.

GaN shows good solubility in supercritical NH₃, provided that alkalimetals or their compounds such as NaNH₂ or KNH₂, are introduced to it.Based on the tests carried out by the inventors, solubility isincreasing with pressure and decreasing with temperature. Based on thoserelationships it is possible to carry out the process according to thisinvention and obtain desired crystals.

Feedstock is placed in the upper zone of the reactor. This zone ismaintained under a different temperature regime from that in the lowerzone of the reactor, wherein at least one mono-crystalline seed isplaced.

In particular, the negative temperature coefficient of GaN solubility inthe process environment means that, as the result of the temperaturegradient, it is possible to evoke the chemical transport of galliumnitride from the upper reactor zone of lower temperature—which is thedissolution zone of the feedstock in the form of crystalline galliumnitride, to the lower zone of higher temperature—which is thecrystallization zone.

The use of crystalline gallium nitride as preferable feedstock in there-crystallization process of GaN is preferable because it assures theamount of gallium as required for the process in the form which iseasily soluble and can be gradually dissolved.

As mentioned above, seeds for crystallization from the supercriticalammonia-containing solution can be obtained by any method. Preferably,GaN crystals are used, which are obtained by HVPE method, by which it ispossible to obtain GaN single crystals in the form of wafers ofrelatively large surface. By using such seeds, bulk monocrystallinegallium-containing nitride which are obtained according to the inventionhave very low dislocation density, but simultaneously they are verythick. The material according to the invention is a perfect material forsubstrates for epitaxial deposition of semiconductor layers. At the sametime, it may be used for preparing seeds for subsequent processes whichare carried out in the way as described above.

As the mineralizer, it is possible to use alkali metals, theircompounds, especially those containing nitrogen and hydrogen and theirmixtures. Alkali metals may be selected from Li, Na, K, Rb and Cs,whereas their compounds may be selected from hydrides, amides, imides,amido-imides, nitrides and azides.

The supercritical environment of ammonia-containing solution withaddition of ions of alkali metals, used for obtaining bulkmono-crystalline gallium-containing nitride according to the inventionmay also contain ions of other metals and soluble forms of otherelements, introduced intentionally to modify the properties of theobtained mono-crystalline gallium-containing nitride. However, thisenvironment contains also incidental impurities which are introducedtogether with the feedstock and released to that environment during theprocess from the elements of applied apparatuses. It is possible toreduce the amount of incidental impurities by the use of high purityreagents, or even additionally purified reagents for the needs of theprocess. Impurities from apparatuses are also subject to control throughselection of construction materials in accordance with the principles asknown to the experts in that field.

Preferably, the controlled growth of crystals according to the inventionin a desired direction, either perpendicular or parallel to the c axis,is carried out by the method as described in detail in the followingexamples, which are illustrated by diagrams of relationships betweentemperature and duration of the process, as presented in the attacheddrawing. As shown in FIG. 2 the temperature, in the crystallizationphase from the supercritical ammonia-containing solution, in the upperzone—which is the dissolution zone of the autoclave as presented brieflyin FIGS. 3 and 4 (and described in more detailed below) is kept lowerthan in the dissolution zone, wherein the temperature is essentiallykept at the constant level during the entire crystallization cycle.

In those conditions—as the result of temperature difference between thezones, and the temperature gradient—dissolution of the feedstock takesplace in the dissolution zone, and the convection leads to chemicaltransport between the zones, and when super-saturation of supercriticalammonia-containing solution is attained with respect to GaN thecrystallization of GaN is carried out on seeds in the crystallizationzone.

During the growth from the supercritical ammonia-containing solution,the value of the temperature difference between the zones may change ina wide range, and preferably, it is from several to several dozensdegree centigrade. Additionally, in accordance with the invention thedifference of temperatures between the zones may be changed during theprocess. In this way it is possible to control the growth rate and thequality of the obtained bulk mono-crystalline gallium-containingnitride.

Additionally, it is possible to modify that basic process, for example,by changing the temperature in both zones periodically, yet, thetemperature in the crystallization zone must be always higher than thetemperature in the dissolution zone.

Authors of the studies over optimization of the flux process which isnow used for the controlled growth of gallium-containing crystals, usedmolybdenum crucibles, as shown in FIG. 1, which are placed in ahigh-temperature reactor with controlled atmosphere, adjusted to workunder increased pressure, and equipped with zonal heating devices. InFIG. 1 the crucible A is filled with the Li—Ga melt containing the abovedefined additional flux (selected from the group of Bi, In, K, Na, Pb,Rb, Sb, Sn and Te). At the bottom of the crucible A there is crystallinefeedstock C in the form of GaN, which is an internal source of nitrogen.The seed B is introduced into the melt at a specified phase of theprocess, and may be lowered into and pulled out from the melt by way ofthe mechanism which is not shown on the drawing. Two orientations of thecrystalline seed show that the zone of growth on the seed may beoriented in various ways within the crucible.

The growth from the supercritical ammonia-containing solution may becarried out in reactors of various structures. In the followingexamples, the autoclave 1 which is schematically shown in FIG. 3 andFIG. 4 was used. The autoclave 1, equipped with the installation 2 inthe form of the baffle, is equipped with two furnaces 3 and 4 equippedwith heating 5 and/or 6 cooling devices. The installation 2 may be inthe form of a horizontal baffle or baffles 7 with central and/orcircumferential openings, which separate the upper dissolution zone 8and the lower crystallization zone 9 in the autoclave 1. The temperaturevalue of each zone in the autoclave 1, in the range of temperature of100 to 800° C., may be set on furnaces 3 and 4 by way of a controllingdevice (not shown in the drawing). In the autoclave 1 the dissolutionzone 8 is above the horizontal baffle or baffles 7 and the feedstock 10is placed in that zone. Whereas the crystallization zone 9 is below thehorizontal baffle or baffles. At least one seed 11 is put in that zone.The place in which that seed 17 is put is below the point ofintersecting the ascending and descending convection flows.

Bulk mono-crystalline gallium-containing nitride according to theinvention is characterized in that it has very low surface dislocationdensity. It may contain alkali metals in amount of about 0.1 ppm ormore—even more than 1.0 ppm, and even more than 10 ppm alkali metals,which are introduced into the system as a flux or mineralizer (dependingon the type of the process of controlled crystal growth in a desireddirection). GDMS (Glow-Discharge Mass Spectroscopy) profiles for aproduct sample according to the invention show the presence of alkalimetals in the range from 0.1 ppm to several ppm. Moreover, sometransition metals (Fe, Cr, Ni, Co, Ti, Mn), present in the reactionenvironment, provide a measurable signal. For comparison, the analogicalprofiles for the GaN crystal obtained by HVPE method show the presenceof potassium in the amount below 0.1 ppm. Whereas the profiles oftransitional metals are present at the noise level, which proves thatthere is a very low amount of those elements in the GaN crystal obtainedby HVPE method.

On the basis of the performed tests the authors of the inventiondetermined the conditions of controlling the process of growing GaNsingle crystals on seeds from Ga—Li melts, in the presence of the abovespecified additional flux and from the supercritical ammonia-containingsolution. Those conditions were positively verified also for nitridescontaining other elements of Group XIII, and for mixed nitridescontaining gallium and other elements of Group XIII. Due to similarparameters of gallium, aluminum and indium nitride crystalline latticesit is possible to partially replace gallium with indium and/or aluminumin gallium-containing nitride obtained according to the invention.

The invention is described in more detail in the following examples.

EXAMPLE 1 Flux Process

A mixture of metallic gallium and lithium was placed in ahigh-temperature reactor (FIG. 1) in a molybdenum crucible (A), havingthe volume of 250 cm3. Additional flux selected from a group consistingof In, K, Na, Pb, Rb, Sb, Sn and Te was also added to the system, insuch an amount that the molar ratio of X:Ga:Li in the performedexperiments between 0.5:1.0:1.5 and 1.5:1.0:2.5. The mixture was heatedto ca. 780° C. in the argon (Ar) atmosphere and—as a result—an alloy ofthe aforementioned metals of the given molar ratio of X:Ga:Li wasobtained. After one day the atmosphere was changed to nitrogen (N2)under the pressure of 2.3 MPa. Such temperature and pressure conditionsin the reactor were then maintained for the next several days. Then theprocess of growth of mono-crystalline gallium nitride on the seedcrystals (B), in the form of mono-crystalline wafers orientedessentially perpendicularly to the c axis of the crystal and having thesurface area of the section perpendicular to the c axis from 0.25 to 4cm², was started. The duration of the growth process at the processconditions was 1-2 weeks. The reactor was then initially slowly cooleddown and then further (fast) cooled down to the room temperature (RT).Alternatively, the seeds were slowly pulled out of the molten alloy atthe process conditions. As the result of the process, the increment ofthe surface area of the seed crystals (measured in the C-plane of thecrystal) by ca. 20% was observed. The GaN single crystals obtained inthe process were stored for further measurements and use.

EXAMPLE 2 Diffusion Flux Process

A mixture of metallic gallium and lithium was placed in ahigh-temperature reactor (FIG. 1) in a molybdenum crucible (A), havingthe volume of 250 cm3. Additional flux selected from a group consistingof In, K, Na, Pb, Rb, Sb, Sn and Te was also added to the system, insuch an amount that the molar ratio of X:Ga:Li in the performedexperiments between 0.5:1.0:1.5 and 1.5:1.0:2.5. The mixture was heatedin the argon (Ar) atmosphere until average temperature of ca. 780° C.was reached, wherein the temperature in the upper part of the cruciblewas by several dozen degrees centigrade lower than the averagetemperature, while the temperature in the lower part of the crucible wasby several dozen degrees centigrade higher than the average temperature.As the result—an alloy of the aforementioned metals of the given molarratio of X:Ga:Li was obtained. After one day the temperature gradient inthe crucible was reversed and the atmosphere was changed to nitrogen(N2) under the pressure of 2.3 MPa. Such temperature and pressureconditions in the reactor were then maintained for the next severaldays. Then the process of growth of mono-crystalline gallium nitride onthe seed crystals (B), in the form of mono-crystalline wafers obtainedas described in Example 1, oriented essentially perpendicularly to the caxis of the crystal and having the surface area of the sectionperpendicular to the c axis from 0.25 to 4 cm2, was started. The seedswere placed in the lower part of the crucible. The duration of thegrowth process at the process conditions was 1-2 weeks. The reactor wasthen initially slowly cooled down and then further (fast) cooled down tothe room temperature (RT). Alternatively, the seeds were slowly pulledout of the molten alloy at the process conditions. As the result of theprocess, the increment of the surface area of the seed crystals(measured in the C-plane of the crystal) by ca. 10% was observed. TheGaN single crystals obtained in the process were stored for furthermeasurements and use.

EXAMPLE 3 Convection Flux Process

A mixture of metallic gallium and lithium was placed in ahigh-temperature reactor (FIG. 1) in a molybdenum crucible (A), havingthe volume of 250 cm3. Additional flux selected from a group consistingof In, K, Na, Pb, Rb, Sb, Sn and Te was also added to the system, insuch an amount that the molar ratio of X:Ga:Li in the performedexperiments between 0.5:1.0:1.5 and 1.5:1.0:2.5. In addition,GaN-containing feedstock (C) was put at the bottom of the crucible. Themixture was heated in the argon (Ar) atmosphere until averagetemperature of ca. 780° C. was reached, wherein the temperature in theupper part of the crucible was by several dozen degrees centigrade lowerthan the average temperature, while the temperature in the lower part ofthe crucible was by several dozen degrees centigrade higher than theaverage temperature. As the result—an alloy of the aforementioned metalsof the given molar ratio of X:Ga:Li was obtained. After one day theatmosphere was changed to nitrogen (N2) under the pressure of 2.3 MPa.Such temperature and pressure conditions in the reactor were thenmaintained for the next several days. Then the process of growth ofmono-crystalline gallium nitride on the seed crystals (B), in the formof mono-crystalline wafers obtained as described in Example 1, orientedessentially perpendicularly to the c axis of the crystal and having thesurface area of the section perpendicular to the c axis from 0.25 to 4cm2, was started. The seeds were placed in the upper part of thecrucible. The duration of the growth process at the process conditionswas 1-2 weeks. The reactor was then initially slowly cooled down andthen further (fast) cooled down to the room temperature (RT).Alternatively, the seeds were slowly pulled out of the molten alloy atthe process conditions. As the result of the process, the increment ofthe surface area of the seed crystals (measured in the C-plane of thecrystal) by ca. 25% was observed. The GaN single crystals obtained inthe process were stored for further measurements and use.

EXAMPLE 4 Crystallization from Supercritical Ammonia-Containing Solution

According to the disclosure of WO 02/101120, dissolution zone of a 600cm³ high-pressure autoclave (FIG. 3 and FIG. 4) was charged withgallium-containing feedstock, seeds, mineralizer and ammonia. The seedswere gallium nitride single crystals, in the form of mono-crystallinewafers obtained as described in Example 1, oriented essentiallyperpendicularly to the c axis of the crystal and having the surface areaof the section perpendicular to the c axis from 0.25 to 4 cm2. Metallicsodium was used as the mineralizer. The feedstock was placed in thedissolution zone, while the seeds were mounted in the crystallizationzone (FIG. 3). The crystallization process on the seeds was carried outunder constant temperature conditions of T2=550° C. in thecrystallization zone and T1=500° C. in the dissolution zone. Thistemperature distribution inside the autoclave was maintained for 16 days(FIG. 2). At such conditions the pressure within the autoclave was ca.390 MPa. As the result of the process, partial dissolution of thefeedstock in the dissolution zone and growth of mono-crystalline galliumnitride layers on both sides of each seed in the crystallization zonewas observed. The total thickness of the re-crystallized layers(measured along the c axis of the crystal) was ca. 1200 μm (on eachseed). The GaN single crystals obtained in the process were stored forfurther measurements and use.

EXAMPLE 5 Crystallization from Supercritical Ammonia-Containing Solution

Procedures as described in Example 4 were followed except that insteadof metallic sodium—a) metallic lithium, b) sodium azide or c) sodiumbromide was used as the mineralizer. After 16 days of the process thegrowth of mono-crystalline gallium nitride layers on both sides of theseed in the crystallization zone was observed. The total thickness ofthe re-crystallized layers (measured along the c axis of the crystal)was ca. a) 380 μm, b) 840 μm and c) 530 μm, respectively. The GaN singlecrystals obtained in the process were stored for further measurementsand use.

EXAMPLE 6 Crystallization from Supercritical Ammonia-Containing Solution

Procedures as described in Example 4 were followed and the seeds usedhad the form of mono-crystalline wafers oriented essentiallyperpendicularly to the c axis of the crystal and having the surface areaof the section perpendicular to the c axis from 0.25 to 4 cm2, while thethese wafers had the shape of squares or isosceles triangles. As theresult of the process, partial dissolution of the feedstock in thedissolution zone and growth of mono-crystalline gallium nitride layerson faces parallel to the c axis of the crystal as well as on C faces ofthe crystal on each seed in the crystallization zone was observed. Thetotal thickness of the re-crystallized layers (measured perpendicularlyto the c axis of the crystal) was ca. 2 mm, while the total thickness ofthe re-crystallized layers (measured along the c axis of the crystal)was ca 1200 μm (on each seed). The GaN single crystals obtained in theprocess were stored for further measurements and use.

EXAMPLE 7 Crystallization from Supercritical Ammonia-Containing Solution

Procedures as described in Example 6 were followed except that sodiumbromide was used as the mineralizer, instead of metallic sodium. Theseeds seed crystals in the form of mono-crystalline wafers orientedessentially perpendicularly to the c axis, having hexagonal shape withsix planes parallel to the c axis of the crystal and having the surfacearea of the section perpendicular to the c axis from 0.25 to 4 cm2 wereused. After 16 days of the process the growth of mono-crystallinegallium nitride layers on all faces parallel to the c axis on each seedin the crystallization zone was observed. The total thickness of therecrystallized layers (measured perpendicularly to the c axis of thecrystal) was ca. 1100 μm. The GaN single crystals obtained in theprocess were stored for further measurements and use.

EXAMPLE 8 Crystallization from Supercritical Ammonia-Containing Solution

Procedures as described in Example 4 were followed except that duringcrystallization step the temperature in the crystallization zone wasT2=500° C. and in the dissolution zone the temperature was T1=450° C.The seeds used had the form of mono-crystalline wafers orientedessentially perpendicularly to the c axis of the crystal and having thesurface area of the section perpendicular to the c axis from 0.25 to 4cm2, while the wafers had the shape of squares or isosceles trianglesand at least one of the side planes was parallel to the A plane of thecrystal. As the result of the process, partial dissolution of thefeedstock in the dissolution zone was observed and growth ofmono-crystalline gallium nitride layers on faces parallel to the c axisof the crystal, of ca. 400 μm in thickness on each seed (measuredperpendicularly to the c axis of the crystal), as well as on both Cfaces of each seed, the total thickness of the re-crystallized layers(measured along the c axis of the crystal) being ca 700 μm on each seed.The GaN single crystals obtained in the process were stored for furthermeasurements and use.

EXAMPLE 9 Crystallization from Supercritical Ammonia-Containing Solution

Procedures as described in Example 6 were followed except that duringre-crystallization stage, after the system reached the targettemperatures of T2=550° C. in the crystallization zone and T1=500° C. inthe dissolution zone, respectively, the temperature T1 in thedissolution zone was being changed periodically in the range 500-450°C., while the temperature T2 in the crystallization zone was beingchanged periodically in the range 550-500° C. in such a way that thecrystallization zone was always the warmer one. This way the growth ofcrystals perpendicular and parallel to the c axis was stimulated. After16 days of the process the growth of mono-crystalline gallium nitridelayers on all faces parallel to the c axis as well as on C faces of thecrystal on each seed in the crystallization zone was observed. The totalthickness of the re-crystallized layers (measured along the a axis, i.e.perpendicularly to the c axis of the crystal) was ca. 900 μm and thetotal thickness of the re-crystallized layers (measured along the c axisof the crystal) was also ca. 900 μm. The GaN single crystals obtained inthe process were stored for further measurements and use.

The crystals which were obtained in examples 1-9 were estimated. Thosecrystals high crystalline quality, low level of crystalline latticedeflection (long curvature radius of wafers), preferable values of FWHMrocking X-ray curve from (0002) below 60 arcsec, and in more preferableexamples (Example 6) below 40 arcsec. The inventors discovered that thesuggested techniques of single crystal growth are not limited either interms of equipment or materials which could essentially lowerpossibilities of obtaining large-sized single crystals.

Use of proper (preferably alternate) combinations of growth of galliumnitride crystal perpendicularly to the c axis (examples 1-3 and 6-9) andparallel to the c axis (examples 4-6 and 8-9) allows to obtaingallium-containing nitride single crystals of the volume of 2.5 cm³ andthe surface face area in C plane of about 5 cm². Such crystals, due totheir high crystalline quality and dimensions, may be sliced intowafers, and then used as the substrate for nitride based opto-electricsemiconductor devices. Such wafer may have any orientation, and haveeither polar or non-polar faces. The cut may be done in a desireddirection with respect to the growth of the single crystal. Inparticular, the cut essentially oriented toward the growth of the singlecrystal assures additional reduction of surface dislocation density.

1-17. (canceled) 18: A bulk mono-crystalline gallium-containing nitride,grown on a seed at least in a direction substantially perpendicular to adirection of seed growth, essentially without propagation of crystallinedefects as present in the seed, having a dislocation density notexceeding 10⁴/cm² and lower compared to a dislocation density of theseed, and having a curvature radius of the crystalline lattice, greaterthan 15 m, and greater than a curvature radius of a crystalline latticeof the seed. 19: The bulk mono-crystalline gallium-containing nitrideaccording to claim 18, wherein the curvature radius of the crystallinelattice of the bulk mono-crystalline gallium-containing nitride isgreater than 30 m. 20: The bulk mono-crystalline gallium-containingnitride according to claim 18, wherein curvature radius of thecrystalline lattice of the bulk mono-crystalline gallium-containingnitride is greater than 70 m. 21: The bulk mono-crystallinegallium-containing nitride according to claim 18, wherein the bulkmono-crystalline gallium-containing nitride is doped with donor-typeand/or acceptor-type, and/or magnetic-type dopants, at a concentrationfrom 10¹⁷/cm³ to 10²¹/cm³, and comprises n-type, p-type or a compensatedmaterial. 22: The bulk mono-crystalline gallium-containing nitrideaccording to claim 18, wherein the bulk mono-crystallinegallium-containing nitride is grown in an environment and in conditionsin which a growth rate in a direction perpendicular to a c axis, andalong an a axis, is equal to or higher than a growth rate in a directionparallel to the c axis of the single crystal. 23: The bulkmono-crystalline gallium-containing nitride according to claim 18,wherein the bulk mono-crystalline gallium-containing nitride comprises agallium nitride single crystal. 24: A bulk mono-crystallinegallium-containing nitride grown on a seed, at least in a directionsubstantially perpendicular to a direction of seed growth, essentiallywithout propagation of crystalline defects present in the seed, the bulkmono-crystalline gallium-containing nitride having a FWHM of an X-rayrocking curve from a (0002) plane, below 40 arcsec for Cu K α1 and lowerthan a FWHM of the seed with a simultaneous curvature radius of thecrystalline lattice, greater than 15 m, and greater than a curvatureradius of the crystalline lattice of the seed. 25: The bulkmono-crystalline gallium-containing nitride according to claim 24,wherein curvature radius of the crystalline lattice of the bulkmono-crystalline gallium-containing nitride is greater than 30 m. 26:The bulk mono-crystalline gallium-containing nitride according to claim24, wherein curvature radius of the crystalline lattice of the bulkmono-crystalline gallium-containing nitride is greater than 70 m. 27:The bulk mono-crystalline gallium-containing nitride according to claim24, wherein the bulk mono-crystalline gallium-containing nitride isdoped with donor-type and/or acceptor-type, and/or magnetic-typedopants, at a concentration from 10¹⁷/cm³ to 10²¹/cm³, and comprisesn-type, p-type or a compensated material. 28: The bulk mono-crystallinegallium-containing nitride according to claim 24, wherein the bulkmono-crystalline gallium-containing nitride is grown in an environmentand in the conditions in which a growth rate in a directionperpendicular to a c axis, and along an a axis, is equal to or higherthan a growth rate in a direction parallel to the c axis of the singlecrystal. 29: The bulk mono-crystalline gallium-containing nitrideaccording to claim 24, wherein the bulk mono-crystallinegallium-containing nitride comprises a gallium nitride single crystal.30: A wafer of any orientation, having polar or non-polar surfaces,obtained as a bulk mono-crystalline gallium-containing nitride, grown ona seed at least in a direction substantially perpendicular to adirection of seed growth, essentially without propagation of crystallinedefects present in the seed, wherein a surface dislocation density ofthe wafer is additionally reduced as a result of slicing in a directionessentially parallel to a direction of growth of the single crystal. 31:The wafer according to claim 30, wherein the wafer hasfurther-processable non-polar surfaces. 32: The wafer according to claim30, wherein the wafer has further-processable polar surfaces. 33: Theuse of the wafer according to claim 30 as a substrate for epitaxialdeposition of semiconductor structures made of nitrides of elements ofGroup XIII. 34: The wafer according to claim 30, wherein the surfacedislocation density, especially at a Group XIII element-terminated side,having an epitaxial surface not smaller than 100 mm². 35: The waferaccording to claim 34, wherein the surface dislocation density,especially at the Group XIII element-terminated side, having anepitaxial surface not smaller than 450 mm². 36: A method of preparing abulk mono-crystalline gallium-containing nitride, grown on a seed atleast in a direction substantially perpendicular to a direction of seedgrowth, essentially without propagation of crystalline defects aspresent in the seed, having a dislocation density not exceeding 10⁴/cm²and lower than the dislocation density of the seed, and having acurvature radius of the crystalline lattice, greater than 15 m, andgreater than a curvature radius of a crystalline lattice of the seed,comprising the steps of: growing the bulk mono-crystallinegallium-containing nitride in a direction perpendicular to a C-axis ofthe seed by a flux method and; growing the bulk mono-crystallinegallium-containing nitride in a direction of the C-axis of the seed bycrystallization from a supercritical ammonia-containing solution. 37:The method of preparing a bulk mono-crystalline gallium-containingnitride according to claim 36, wherein said steps produce the bulkmono-crystalline gallium-containing nitride having a curvature radius ofthe crystalline lattice greater than 30 m. 38: The method of preparing abulk mono-crystalline gallium-containing nitride according to claim 36,wherein said steps produce the bulk mono-crystalline gallium-containingnitride having a curvature radius of the crystalline lattice greaterthan 70 m. 39: The method of preparing a bulk mono-crystallinegallium-containing nitride according to claim 36, wherein said step ofgrowing the bulk mono-crystalline gallium-containing nitride in adirection perpendicular to a C-axis of the seed by a flux method is usedfor growing the bulk mono-crystalline gallium-containing nitride in adirection of an A or M-axis of the seed. 40: The method of preparing abulk mono-crystalline gallium-containing nitride according to claim 36,wherein said growing steps are repeatedly carried out. 41: The method ofpreparing a bulk mono-crystalline gallium-containing nitride accordingto claim 36, wherein said step of growing the bulk mono-crystallinegallium-containing nitride in a direction perpendicular to a C-axis ofthe seed by a flux method is a method of growing the bulkmono-crystalline gallium-containing nitride from a liquid mixture ofmetals containing lithium and gallium together with additional fluxselected from the group consisting of Bi, In, Pb, Rb, Sb, Sn and Te.